IMS Push-to-talk over Cellular (PoC) Call Flow Diagrams

Push-to-talk over Cellular (PoC) service allows cell phones
to used as walkie-talkies. A group of users in a PoC session can communicate
by simply pressing a button and speaking when the phone indicates it is OK
to do so. The user releases the button when he or she is done speaking.

When a user begins to speak, the PoC server allocates
resources and notifies other users in the PoC session that the user is
speaking. The PoC server then delivers the speech packets to all the users
in the session.

PoC is resource efficient as it allocates resources only
when a user is actually speaking. This makes it suitable for applications
where there are long gaps between individual session participants speaking.

This flows covers the case where PoC Client A invites PoC
Client B to a Pre-established Session by sending SIP REFER request to PoC
Server A.